The present invention relates to a CCD shift register, and more specifically to a two-phase CCD shift register which can be advantageously used as a horizontal shift register of a solid-state image sensor.
FIG. 6 shows one conventional horizontal CCD shift register of an image sensor. This shift register is formed in and on a portion of a semiconductor substrate 11. The CCD shift register has a charge transfer section 15 including a series of transfer electrodes, an output gate section 16, and a charge detecting section including a floating diffusion region 21. Each transfer electrode consists of a first polysilicon gate electrode 13 and a second polysilicon gate electrode 14. Under the transfer electrodes, there is formed a potential adjusting p-type region 61 for lowering the potential level in the whole of the charge transfer section. The substrate further includes an n-type region under the second gate electrode 14 of each transfer electrode. This structure can provide a potential profile as shown in FIG. 7. In this potential profile, it is necessary to make a drive bias of the final transfer stage of the charge transfer section 15 shallower than the "L" level, to transfer signal charges from the transfer section 15 to the floating diffusion region 21. On the other hand, the potential level of the output gate (OG) section 16 need not be made so shallow. It is sufficient to obtain such a level as to ensure a dynamic range of the floating diffusion region 21. Rather, the structure that does not allow free potential adjustment of the output gate section is disadvantageous.
In this conventional CCD shift register, adjustment of an impurity concentration of the output gate section incurs mask misalignment. Therefore, it is difficult to adjust the potential level of the output gate section by adjusting the impurity concentration. It is necessary to adjust an external bias voltage VOG, to adjust the potential of the output gate section.